In the past, an internal combustion engine which is provided with an exhaust purification catalyst in an exhaust passage of an internal combustion engine, and which is provided with an air-fuel ratio sensor at an upstream side, in the direction of flow of exhaust, of the exhaust purification catalyst and an electromotive force type oxygen sensor at the downstream side of the exhaust purification catalyst, has been widely known. In such a control system of an internal combustion engine, the amount of fuel fed to the internal combustion engine is controlled based on the outputs of these air-fuel ratio sensor and oxygen sensor.
However, in an electromotive force type oxygen sensor, the output for the same air-fuel ratio is different between when the air-fuel ratio of the exhaust gas around the oxygen sensor changes from an air-fuel ratio which is richer than the stoichiometric air-fuel ratio (below, “rich air-fuel ratio”) to an air-fuel ratio which is leaner than the stoichiometric air-fuel ratio (below, “lean air-fuel ratio”) and when it changes from a lean air-fuel ratio to a rich air-fuel ratio. Therefore, it has been proposed to use a limit current type air-fuel ratio sensor at the downstream side of the exhaust purification catalyst (for example, PTL 1).
However, even if using a downstream side air-fuel ratio sensor, sometimes deviation occurs in the output due to aging or initial variations, etc. Therefore, in the control system described in PTL 1, deviation in the downstream side air-fuel ratio sensor is corrected. Specifically, in the control system described in PTL 1, active air-fuel ratio control is performed so as to alternately switch the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst between the rich air-fuel ratio and the lean air-fuel ratio. In addition, during this active air-fuel ratio control, the output of the air-fuel ratio sensor is corrected in accordance with the difference between the output of the downstream side air-fuel ratio sensor and the reference output which corresponds to the stoichiometric air-fuel ratio, in a predetermined time period where the output of the downstream side air-fuel ratio sensor becomes balanced. According to PTL 1, due to this, it is considered possible to correct deviation due to the degradation of the downstream side air-fuel ratio sensor, etc.